Nd—Fe—B-based rare earth sintered magnet was invented by Sagawa being an inventor of the present application et al. in 1982, and its characteristics far outperforms conventional permanent magnets materials and is widely put to practical use (Patent Document 1). Particularly, it has been widely used for a compressor of an air conditioner, a motor or an electric generator of a hybrid car, and a voice coil motor (VCM) of a hard disc, and it helps in downsizing of equipment and saving energy, and contribute to prevention of global warming. Shapes of the rare earth sintered magnets used in these applications are a straight flat plate shape, a curved arc segment plate shape, a sectorial flat plate shape and the like. These plate-shaped rare earth sintered magnet is a thin-walled article in which a thickness in an orientation direction is small compared with a vertical or horizontal length of a plate. In addition, as the rare earth sintered magnet, Sm—Co-based magnet is put to practical use in addition to Nd—Fe—B-based magnet. Hereinafter, both magnets are collectively referred to as a “rare earth sintered magnet”. Sometimes the Nd—Fe—B base includes another rare earth element such as Pr or Dy, but in the present specification, these are generically referred to as a Nd—Fe—B base.
A rare earth alloy powder (hereinafter, referred to as a “alloy powder”) serving as a material of a rare earth sintered magnet is very chemically active, and is not only rapidly oxidized to be degraded, but also ignites sometimes in the atmosphere, and therefore the alloy powder has to be handled in an inert gas atmosphere not containing oxygen. Thus, a rational production process for producing a rare earth sintered magnet from the alloy powder is desired.
As a method for producing a thin-shaped rare earth sintered magnet, two methods are heretofore known. One method is a metallic mold pressing method in which an alloy powder is filled into a metallic mold and press formed in a magnetic field to prepare an molded powder compact and the molded powder compact is sintered (Non-patent Document 1), and another method is a press-less process in which an alloy powder is filled into a filling container (hereinafter, referred to as a “mold”) and oriented by a pulsed magnetic field to obtain an oriented filled-molded-body and the oriented filled-molded-body is sintered remaining housed in the mold (hereinafter, referred to as a “PLP method”) (Patent Document 2).
In the metallic mold pressing method, since it is difficult to press form a thin-walled product, a large block-like molded powder compact is prepared first using a large metallic mold, and the molded powder compact is retrieved from the metallic mold and sintered to obtain a block-like sintered body. The large block-like sintered body is sliced with a peripheral edge cutting machine to form a thin-walled plate-like product. A slicing step costs a great deal, and a large amount of chips are generated during the slicing step and this reduces yields of a raw material (a ratio of a product amount actually achieved to a product amount expected from a raw material input). Therefore, the metallic mold pressing method has the disadvantage that a product price is increased.
Technical contents and problems of the metallic mold pressing method are described in detail in paragraphs [0002] to [0042] of Patent Document 3.
In the metallic mold pressing method, a metallic mold is placed between magnetic poles for a static magnetic field, and an alloy powder is charged into the metallic mold (Patent Document 4). After charging the alloy powder, an upper punch is lowered and a lower punch is simultaneously raised to apply a pressure to the alloy powder between the upper and lower punches while applying a magnetic field, and thereby a molded powder compact can be obtained. If the upper and lower punches are raised, the molded powder compact can be retrieved from the metallic mold. The molded powder compact is sintered to obtain a block-like sintered body.
In the PLP method, it is common to dispose partitions in the mold to produce a plurality of products simultaneously. An alloy powder is charged into a plurality of cavities defined by a plurality of partitions, covered with a lid, and the alloy powder is oriented by applying a pulsed magnetic field, and the obtained oriented filled-molded-body housed in the mold is sintered with the mold (Patent Document 2). By this method, a thin-walled plate-like rare earth sintered magnet with less bending can be produced with efficiency. Since this method achieves a high raw material yield and can reduce process costs, it comes to be employed in mass-production factories.
As the mass production technology of the rare earth magnet, the PLP method has the following problems.
(1) Since the mold is used during sintering, a lot of molds are required. The reason for this is that as the mass production technology, it takes several tens of hours to undergo the sintering step, but it takes only about 5 minutes to undergo the powder feeding/filling/orienting steps.(2) Since the mold has to be made precisely, it takes processing cost. Mold manufacturing cost is expensive.(3) Since the mold is used for mass production, it is assumed that the mold is used repeatedly. In order to use the mold repeatedly, a material of a container portion or a partition constituting the mold must be selected and a thickness thereof must be adequately large. When a wall thickness is increased, material cost is increased, and an occupied volume of the mold in the process step increases, and the productivity per each device from a powder filling device, a powder magnetic field orienting device to a sintering device is lowered.(4) Since the mold is exposed to a high sintering temperature, it reacts with an alloy powder more than a little to be depleted whichever material the mold is made of. Therefore, the mold cannot be permanently used, the number of uses is limited, and mold cost is increased.(5) When the mold is made of a metal, the thicknesses of portions of the mold can be reduced; however, since a metal is easily deformed during sintering at elevated temperatures, there is a limit on repeated use. Therefore, the efforts of decreasing a particle size of the alloy powder and lowering a sintering temperature are made (Patent Document 5); however, the deformation of the metal mold cannot be suppressed. Further, the metal mold easily reacts with the alloy powder, and therefore it is necessary to apply a ceramic powder to the mold every time before filling an alloy powder into the mold (Patent Document 6), and this increases a product price.(6) When the thickness of the partition is increased in order to make the mold robust, variations in the amount of feeding of the alloy powder into the cavity defined by the partition easily occurs, resulting in the occurrence of variations in product dimensions.